Obsidian

Obsidian, a volcanic glass, usually of rhyolitic
composition, forms by rapid cooling of a viscous lava. Most
obsidians are more than 70 percent silica and are low in
volatile contents. Microscopic crystals of quartz or
feldspar are sometimes included in the glass. Obsidian
occurs as thick, short flows or domes over volcanic vents.
It is usually black in color but occasionally red or brown
(if iron-oxide dust is present), clear, or green. Obsidian
displays a well developed conchoidal fracture, which
makes it an excellent material for arrowheads, knives, and
other sharp tools and weapons. Archaeologists use obsidian
tools to trace trade routes, because such tools are
relatively rare and each occurrence has a slightly
different chemical composition. Thus the source of
primitive obsidian tools may be located even if the tools
have been traded across thousands of kilometers.

Obsidian
has also been used as a semiprecious gem because of its
shiny luster. Perlite, a hydrous form of obsidian, is used
as a lightweight aggregate. When heated it expands into an
artificial pumice like material. Its potential expansion
after transport is a major shipping advantage. The rounded
nodules of obsidian left after hydration and alteration of
surrounding material into perlite are known as Apache
tears. Looking like black teardrops, they are collector's
items in the American Southwest. Now used in rock wool.